The cover is dedicated to the Sky-Watcher DOB 12 "Flex telescope, i.e. to the retractable version. The black material with sewn ribs is fastened with Velcro on the outside of the telescope's sliding tubes. The cover eliminates the problem of diffused light falling into the tube and protects to some extent against condensation inside the tube.
(Sky-Wacher Dobson 10" + Light Shround - photo by courtesy of our customer from Germany)